Field of the Invention
The present invention relates to a work vehicle, in particular, a hybrid-type work vehicle, and to a method for controlling the same.
Background Information
Recently, a hybrid-type work vehicle has been proposed that travels using driving power from an engine and driving power from a motor. A hydraulic-mechanical transmission (HMT) or an electric-mechanical transmission (EMT) are disclosed as power transmission devices for hybrid-type work vehicles in Japanese Unexamined Patent Application Publication No. 2006-329244 for example.
The HMT has a planetary gear mechanism, and a first pump/motor and a second pump/motor connected to rotating elements of the planetary gear mechanism. The first pump/motor and the second pump/motor function as either hydraulic motors or hydraulic pumps in response to the travel state of the work vehicle. The HMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the pump/motors.
An electric motor is used in the EMT in place of the hydraulic motor in the HMT. That is, the EMT has a first generator/motor and a second generator/motor. The first and second generator/motors function as either electric motors or electric generators in response to the travel state of the work vehicle. Similar to the HMT, the EMT is configured to enable stepless changing of the rotation speed of the output shaft by changing the rotation speed of the generator/motors.
The above-mentioned hybrid-type power transmission device exhibits less internal loss in the power transmission device in comparison to a conventional power transmission device (referred to below as a “torque converter-type speed change device”) having a torque converter and a multi-stage speed change device. Therefore, the hybrid-type power transmission device exhibits superior efficiency when obtaining tractive force by transmitting driving power from the engine to a travel device and also has the advantage of good fuel consumption.
SUMMARY The conventional work vehicle equipped with the torque converter-type speed change device is able to obtain a braking force from an engine brake. In this case, a portion of the power (referred to below as “braking power”) absorbed by braking is discarded as heat in the torque converter and the remainder is absorbed by the engine.
Conversely, the hybrid-type power transmission device discards less of the braking power as heat due to the above-mentioned superiority of the efficiency. As a result, a large portion of the braking power is returned to the engine. In this case, if the engine attempts to absorb all of the braking power, there is a possibility that the engine rotation speed may rise excessively.
If the braking power absorbed by the engine is limited, the excessive rise in the engine rotation speed may also be suppressed, but in this case there is a problem that the braking force obtained by the engine brake is reduced.
A hybrid-type work vehicle equipped with an electrical power storage device such as a capacitor is able to store a portion of the braking power as electrical energy by causing a generator/motor to generate electricity from the braking power. However, when the electrical power storage device enters a fully charged state, a portion of the braking power can no longer be stored as electrical energy and thus the braking force obtained with the engine brake is reduced.
An object of the present invention is to provide a hybrid-type work vehicle that is able to obtain a large braking force while suppressing an excessive rise in the engine rotation speed during braking, and a control method for the hybrid-type work vehicle.
A work vehicle according to a first exemplary embodiment of the present invention is equipped with an engine, a hydraulic pump, a travel device, a power transmission device, a power take-off device, and a control unit. The hydraulic pump is driven by the engine. The travel device is driven by the engine. The power transmission device transmits driving power from the engine to the travel device. The power take-off device distributes the driving power from the engine to the hydraulic pump and the power transmission device. The control unit controls the hydraulic pump and the power transmission device.
The power transmission device has an input shaft, an output shaft, a gear mechanism, and a motor. The gear mechanism has a planetary gear mechanism and transmits the rotation of the input shaft to the output shaft. The motor is connected to the rotating elements of the planetary gear mechanism. The power transmission device is configured to change a rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speed of the motor.
The control unit has a pump brake control determining unit and a pump brake torque control unit. The pump brake control determining unit determines whether to execute of a pump brake control for causing a braking force to be generated by using a load on the hydraulic pump during braking. The pump brake torque control unit increases the pump brake torque that corresponds to the load on the hydraulic pump during the pump brake control.
In this case, the pump brake torque corresponding to the load on the hydraulic pump is increased when the pump brake control is executed. Therefore, the braking power to be distributed to the hydraulic pump by the power take-off device is increased. As a result, a large braking force can be obtained while suppressing an excessive rise in the engine rotation speed during braking.
The pump brake control determining unit preferably determines that the pump brake control is to be executed when an engine regenerative torque is equal to or greater than a predetermined torque threshold. The engine regenerative torque is a torque regenerated in the engine from the travel device via the power transmission device.
In this case, the pump brake torque can be increased when the engine regenerative torque is equal to or greater than the predetermined torque threshold. For example, the pump brake torque can be increased when an engine regenerative torque that is larger than the torque that can be absorbed by the engine is generated. As a result, an excessive rise in the engine rotation speed can be suppressed.
The pump brake control determining unit preferably determines that the pump brake control is to be executed when the engine rotation speed meets or exceeds a predetermined rotation speed threshold. In this case, an excessive rise in the engine rotation speed can be suppressed.
The pump brake torque control unit preferably controls the pump brake torque on the basis of the engine regenerative torque. In this case, the magnitude of the pump brake torque can be suitably controlled in response to the size of the engine regenerative torque.
The work vehicle preferably is further provided with an energy reservoir unit for storing energy regenerated by the motor. The control unit further has a target braking power determining unit for determining a target braking power, and a reservoir power computing unit for computing a reservoir power of the energy reservoir unit. The target braking power determining unit determines the target braking power. The reservoir power computing unit computes the reservoir power of the energy reservoir unit. The pump brake torque control unit determines the engine regenerative torque on the basis of the target braking power and the reservoir power.
In this case, the braking force can be suitably controlled by determining the target braking power. Further, energy stored in the energy reservoir unit can be ensured by determining the engine regenerative torque on the basis of the target braking power and the reservoir power.
The pump brake torque control unit preferably controls the pump brake torque so that the engine rotation speed meets a target engine rotation speed while the pump brake torque is being generated. In this case, an excessive rise in the engine rotation speed can be suppressed.
The pump brake torque control unit preferably increases the pump brake torque when the vehicle speed reaches or exceeds a predetermined vehicle speed threshold. In this case, the braking force can be increased when the vehicle speed is equal to or greater than the predetermined vehicle speed threshold. As a result, an excessive rise in the vehicle speed can be suppressed.
The work vehicle is preferably further provided with a speed change operating member for selecting a speed range that defines an upper limit of the vehicle speed. The pump brake torque control unit determines the predetermined vehicle speed threshold on the basis of the speed range selected with the speed change operating member. In this case, the braking force can be increased when the vehicle speed is equal to or greater than the predetermined vehicle speed threshold according to the selected speed range. As a result, the braking force can be increased when, for example, the vehicle speed exceeds the selected speed range.
The work vehicle preferably is further provided with a forward/reverse travel operating member for switching between forward travel and reverse travel of the vehicle. The pump brake torque control unit determines the predetermined vehicle speed threshold on the basis of the selection with the forward/reverse travel operating member. In this case, the braking force can be increased when the vehicle speed is equal to or greater than the predetermined vehicle speed threshold according to the selected travel direction.
The work vehicle preferably is further provided with a work implement having a hydraulic cylinder. The hydraulic pump discharges hydraulic fluid for driving the hydraulic cylinder. In this case, the pump brake torque can be controlled by controlling the load on the hydraulic pump for driving the work implement.
The work vehicle is preferably further provided with a relief valve and a pump brake control valve. The relief valve is provided parallel to the hydraulic cylinder in a hydraulic circuit. The pump brake control valve controls the hydraulic fluid supplied to the relief valve. The pump brake torque control unit increases the load on the hydraulic pump by controlling the pump brake control valve. In this case, a gain in the load on the hydraulic pump is discarded as heat of the hydraulic fluid in the relief valve. As a result, the load on the hydraulic pump can be increased while suppressing any adverse effects on the operation of the work implement.
The work vehicle is preferably further provided with a work implement operating member for operating the work implement. The control unit further has a required pump flow rate determining unit and a work implement requirement determining unit. The required pump flow rate determining unit determines a required flow rate of the hydraulic pump on the basis of the pump brake torque. The work implement requirement determining unit determines a required flow rate of the hydraulic cylinder on the basis of the operating amount of the work implement operating member. The pump brake torque control unit determines a flow rate of the pump brake control valve on the basis of the required flow rate of the hydraulic pump and the required flow rate of the hydraulic cylinder.
In this case, a required flow rate of the hydraulic cylinder required for the operation of the work implement can be ensured. The flow rate of the pump brake control valve can be suppressed by considering the deficient portion of the required flow rate for the hydraulic cylinder as the flow rate of the pump brake control valve in order to obtain a desired pump brake torque. As a result, an excessive rise in the temperature of the hydraulic fluid can be suppressed.
The work vehicle preferably is further provided with a cooling fan for cooling cooling water for the engine, and a hydraulic motor for driving the cooling fan. The hydraulic pump discharges hydraulic fluid for driving the hydraulic motor. The pump brake torque control unit increases a load on the hydraulic pump by increasing the rotation speed of the hydraulic motor. In this case, the pump brake torque can be controlled by controlling the load on the hydraulic pump for driving a fan motor.
The work vehicle preferably is further provided with a warm-up hydraulic circuit connected to the hydraulic pump. The pump brake torque control unit increases the load on the hydraulic pump by executing a warm-up operation to increase the temperature of the hydraulic fluid using the warm-up hydraulic circuit. In this case, the pump brake torque can be controlled by controlling the load on the hydraulic pump for executing the warm-up operation.
A control method for a work vehicle according to a second exemplary embodiment of the present invention is a control method for a work vehicle equipped with an engine, a hydraulic pump, a travel device, a power transmission device, and a power take-off device. The hydraulic pump is driven by the engine. The travel device is driven by the engine. The power transmission device transmits driving power from the engine to the travel device. The power take-off device distributes the driving power from the engine to the hydraulic pump and the power transmission device.
The power transmission device has an input shaft, an output shaft, a gear mechanism, and a motor. The gear mechanism has a planetary gear mechanism and transmits the rotation of the input shaft to the output shaft. The motor is connected to the rotating elements of the planetary gear mechanism. The power transmission device is configured to change the rotation speed ratio of the output shaft with respect to the input shaft by changing the rotation speed of the motor.
The control method according to the second exemplary embodiment includes a first step and a second step. In the first step, the execution of a pump brake control for causing braking force to be generated by using a load on the hydraulic pump during braking is determined. In the second step, the pump brake torque is increased to correspond to the load on the hydraulic pump during the pump brake control.
In this case, the pump brake torque corresponding to the load on the hydraulic pump is increased when the pump brake control is executed. Therefore, the braking power to be distributed to the hydraulic pump by the power take-off device is increased. As a result, a large braking force can be obtained while suppressing an excessive rise in the engine rotation speed during braking.